1. Field of the Invention
The present invention relates to a read head with a dual tunnel junction sensor and more particularly to a dual tunnel junction sensor that produces a double tunnel junction effect and has a ferromagnetic free layer structure that has improved linear bit density.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotatable magnetic disk, write and read heads that are suspended by a suspension arm above the disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are mounted on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent the ABS to cause the slider and the write and read heads to ride on an air bearing a slight distance from the surface of the rotating disk. During rotation of the disk the write head writes magnetic bits of information (signal fields) to the disk and the read senses the magnetic bits (signal fields) from the disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A magnetic gap is formed between the first and second pole piece layers by a write gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field across the gap between the pole pieces. This field fringes across the gap at the ABS for the purpose of writing the aforementioned magnetic bits in circular tracks on the rotating disk.
A typical sensor employed by recent read heads for sensing signal fields from the rotating magnetic disk is a spin valve sensor. The spin valve sensor includes a nonmagnetic spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the ABS and the magnetic moment of the free layer is located parallel to the ABS but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer. The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered in phase by the interfaces of the spacer layer with the pinned and free layers. When the magnetizations of the pinned and free layers are parallel scattering is at a minimum and when the magnetizations of the pinned and free layers are antiparallel, scattering is at a maximum. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos .theta., where .theta. is the angle between the magnetizations of the pinned and free layers. When a sense current is conducted through the spin valve sensor in a direction parallel to surface planes of the layers resistance changes cause potential changes that are detected and processed as playback signals by the processing circuitry.
Another type of sensor is a tunnel junction sensor which receives a tunneling current perpendicular to the surface planes of the layers. The tunneling junction sensor includes a nonmagnetic nonconductive spacer layer between a ferromagnetic pinned layer and a ferromagnetic free layer. The spacer layer, which is an oxide, is thin enough that electron tunneling occurs between the free and pinned layers. The resistance of the sensor is spin dependent which means that the resistance of the sensor changes as a function of the relative orientation of the magnetic moments of the free and pinned layers. The pinned layer is located on and exchanged coupled to an antiferromagnetic pinning layer which pins a magnetic moment of the pinned layer in a first direction which is typically perpendicular to the ABS. The free layer has a magnetic moment which is free to rotate in response to signal fields from the rotating disk. A tunneling current I.sub.T tunnels through the oxide spacer layer as a function of sin.sup.2.theta., where .theta. is the angle between the magnetic moments of the pinned and free layers. When the magnetic moments of the free and pinned layers are parallel the resistance to the tunneling current is at a minimum, and when these moments are antiparallel the resistance to the tunneling current is at a maximum. Accordingly, as the tunneling current I.sub.T is conducted through the tunnel junction sensor increases and decreases in the resistance of the sensor causes potential changes that are processed by the aforementioned processing circuitry as playback signals. The processing circuitry employs these potential changes to produce readback signals. The details of tunnel junction are described in a commonly assigned U.S. Pat. No. 5,650,958 to Gallagher et al., which is incorporated by reference herein.
Efforts continue to increase the magnetoresistive coefficient dr/R of spin valve and tunnel junction sensors. One way to increase the magnetoresistive coefficient dr/R is to increase the linear bit density of the sensor. A need exists to provide a tunnel junction sensor that produces asymmetric read back signals.